Ultra high voltage bushings of prior art comprise an electric conductor made of a hollow aluminum tube. Said tube has a large cross-section in order to reduce electric losses during operation, when an electric current is conducted through the conductor. The conductor is surrounded by an insulator, and a gas-filled space is provided between the inner periphery of the insulator and the conductor. In the opposite ends of the bushing, the conductor is mounted in and supported by the insulator. The gas in said space is, preferably, an electrically insulating inert gas such as SF6, and said space is therefore, preferably, gas-sealed. The conductor should have a relatively large outer diameter in order to permit the insulating gas to absorb heat from the conductor, and for the purpose of providing a sufficiently high rigidity of the conductor. It is a design challenge to make the conductor able of coping with elevated mechanical loads, such as those appearing in connection to, for example, an earth quake or any other seismic phenomena, and still being able of carrying high voltages.
Bushings of prior art are well suited for their purpose as long as the electric current to be conducted by the conductor is at a moderate level, i.e. those levels encountered by corresponding bushings in contemporary electric power installations. However, if the electric current is increased, which will most probably be the case in future applications, there will be spots along the length of the conductor where the cooling thereof by the surrounding gas or any other surrounding media is insufficient, thereby resulting in increased losses.
In bushings provided with a condenser of solid material in the space between the inner periphery of the insulator and the conductor there has been suggested to cool the conductor by means of circulation of a liquid such as water in the central channel of the conductor. However, such a solution will add considerable weight, caused by the liquid, to the bushing. In some bushings this is fully acceptable, but in other bushings, such as those in which there is no supporting condenser, it will be unacceptable, since it will make the load on the conductor to large. Accordingly, the conductor will become less able of standing the test of seismic abnormalities.